1. Field of the Invention
The invention relates to a multilayered substrate for a semiconductor device. More particularly, the invention relates to a multilayered substrate formed of a plurality of sets of a conductor layer and an insulation layer, i.e., a laminate of alternate conductor and insulation layers, and having a face for mounting semiconductor element thereon and another face for external connection terminals, the face for mounting semiconductor device being provided with pads through which the substrate is connected to a semiconductor element to be mounted thereon, and the face for external connection terminals being provided with pads through which the substrate is connected to an external electrical circuit. The invention also relates to a method of manufacturing such a multilayered substrate.
2. Description of the Related Art
A typical multilayered substrate for semiconductor device, which may be simply called a multilayered substrate herein, is illustrated in FIG. 19. This multilayered substrate 100 has a core substrate 102 made of resin, and an upper layered section 106a and a lower layered section 106b provided on the respective sides of the core substrate 102, each of the upper and lower layered sections 106a and 106b having layered wiring lines 104 of a conductor, and the wiring lines 104 of the upper and lower layered sections 106a and 106b being electrically connected through a through hole via 110 piercing through the core substrate 102. Also in each of the upper and lower layered sections 106a and 106b, wiring lines 104 on both sides of an insulation layer of resin are electrically connected through a via 118 piercing the insulation layer.
In the outermost layer of the upper layered section 106a, pads 124 are provided which are electrically connected with an electrode terminal 122 of a semiconductor element 120 to be mounted on the multilayered substrate 100. The outermost layer of the upper layered section 106a is covered by a solder resist 126 except for the pads 124.
In the outermost layer of the lower layered section 106b, pads 130 are formed to which a solder ball 128, as an external connection terminal, is to be bonded. The outermost layer of the lower layered section 106b is also covered by a solder resist 132 except for the pads 130.
The multilayered substrate for semiconductor element shown in FIG. 19 can be produced by a build-up process illustrated in FIGS. 20A to 20F. According to the build-up process, the upper and lower layered sections 106a and 106b shown in FIG. 19 are simultaneously formed. For this reason, FIGS. 20A to 20F show only the formation of the upper layered section 106a, omitting the formation of the lower layered section 106b. 
In the process shown in FIGS. 20A to 20F, a core substrate of resin 102 having a copper foil 100 on each side is first pierced by a means such as a drill to be provided with through holes. A copper film layer is then formed on the inside wall of the through hole by electroless plating and, as required, by subsequent electrolytic plating, to have a desired thickness and form a through hole via 110 (FIG. 20A).
The copper foil 100 of the core substrate 102 is then subjected to a subtractive process to form wiring lines 104 and pads 105, which are formed as part of the wiring line (FIG. 20B). A film 106 of polyimide, which is a thermosetting resin, having one face provided with a copper foil 108 thereon is adhered to the core substrate 102, with the face provided with the copper foil 108 being upwardly faced (FIG. 20C). In the step of adhesion of the polyimide film 106, a resin, such as a polyimide resin, is filled in the through hole vias 110. The film 106 with the copper foil 108 is then pierced by a laser beam to form holes 107 for the formation of vias, the hole extending to the underlying wiring line 104 to expose the pad 105 (FIG. 20D).
A copper layer 112 is then formed on the inside wall of each of the holes 107 to electrically connect the pad 105 of the wiring line 104 and the copper foil 108 (FIG. 20E). The copper layer 112 is formed by covering the copper foil 108 by a resist film 114 to leave the holes 107 uncovered, as shown in FIG. 20E, and forming a copper film layer, having a certain thickness, only on the inside wall of each hole 107 by electroless plating, sputtering or the like and, as required, by subsequent electrolytic plating.
The resist film 114 is then removed, and the copper foil 108 is subjected to a subtractive process to form wiring lines 116 (FIG. 20F). The wiring line 116 thus formed is electrically connected with the underlying wiring line 114 through a via 118 penetrating the resin layer 106.
By repeating the steps illustrated in FIGS. 20C to 20F, the multilayered substrate for a semiconductor, shown in FIG. 19, can be obtained.
Using the multilayered substrate 100 as shown in FIG. 19, a semiconductor element 120 having a high density can be mounted thereon. In the upper layered section 106a of the multilayered substrate 110 of FIG. 19, however, since the respective layers are successively formed upwardly from the surface of the core substrate 102, the outermost layer, on which a semiconductor element 120 is to be mounted, is prone to have a less flat surface (i.e., a less even surface) due to accumulation of unevenness of the underlying layers. Consequently, when a semiconductor element 120 is mounted on the mounting face of the multilayered substrate of FIG. 19 in a flip chip fashion, for example, some of electrode terminals 122 of the semiconductor element 120 may be left unconnected to the pads 124 of the outermost layer of the substrate 100.
In a build-up process illustrated in FIGS. 20A to 20F, the respective layers of the upper and lower layered sections 106a and 106b are simultaneously built up on the respective sides of the substrate, as described above, because if layers are formed only on one side of the substrate, the resultant substrate may be warped. Consequently, even if it is sufficient for a substrate to have only upper layered section, it is required to form the lower layered section to prevent the substrate from being warped, which makes the resultant substrate thicker.